The construction of new carbon-carbon bonds is central to the synthesis of the complex molecules often found to display useful or interesting biochemical or physiological activity. Organotransition metal complexes continue to offer versatile new approaches to bond formation. Metallacyclobutenes react with unsaturated organic substrates such as carbonyl compounds, nitriles, and sulfoxides, yielding organometallic complexes which may be transformed into a wide variety of important heterocyclic organic molecules. The generality and functional group tolerances of these transformations will be explored. Further synthetic applications will be achieved through the synthesis of a series of new metallacyclobutenes, in which systematic variation of the transition metal and of its supporting ligands will permit both coarse and fine tuning of metallacycle reactivity and substrate specificity.